Internal view of the rear leveling valve.

[coheed]

As I mentioned in the thread I made about just bleeding the rear brakes I had a crappy old leveling valve that wasn't really worth keeping. Today I took it to work and cut it open on the bandsaw. I didn't realize the blade guides were loose so the cut was a little crooked, but it's still a pretty decent cross section of the valve and all the bits therein. 

 

I'm pretty positive that the little green valve piece is oriented the way I did it in the picture, but someone more knowledgeable of brake valves may correct me. If it is oriented that way it has about 3/32 of back and forth that would change how much pressure is let through. The spring got absolutely destroyed and that was the largest piece I could recover.

[gvthnks]

Interesting.   Mine is still intact, but I have no idea if it's really functional.   

 

[Ωhm]

2 hours ago, coheed said:

If it is oriented that way it has about 3/32 of back and forth that would change how much pressure is let through

 

I think (open for discussion here) that the pressure stays the same everywhere in the circuit (both front and rear), but the valve will add more fluid (volume) to the rear brakes allowing the wheel cylinders to fill faster making for a quicker braking response.

 

Thanks for the inside view of this mysterious valve.

[coheed]

1 hour ago, Ωhm said:

 

I think (open for discussion here) that the pressure stays the same everywhere in the circuit (both front and rear), but the valve will add more fluid (volume) to the rear brakes allowing the wheel cylinders to fill faster making for a quicker braking response.

 

Thanks for the inside view of this mysterious valve.

That's a fair point. I forgot brake fluid is non-compressible so the pressure is the same throughout the system. Also, even with the opened valve in hand I'm still not sure how the fluid gets through the valve.

[gogmorgo]

I stumbled across this thread earlier this week and have spent waaaaay too much time thinking about this. The fluid flow through the valve, I mean.

I have to make some assumptions based on not having the pieces at hand, but the result is a fairly standard sort of adjustable pressure regulating valve.

While thinking about how the valve works, remember there's two modes of operation of the valve.

The first dynamic mode is while fluid is flowing, when you step on the brake pedal and the wheel cylinders fill and push the shoes into the drums. Fluid pressure is not going to be the same through all parts of the system while fluid is flowing. Pressure will be lowest at the wheel cylinder end, and every little restriction, valve, orifice, kink in a line, is going to create a pressure spike as fluid resists moving through the restriction, up to the highest pressure area in the master cylinder.

The second mode is while you're holding steady pressure on the pedal and everything is static. The shoes have hit the drums and fluid is no longer flowing to expand the wheel cylinders. In static mode, pressure will be more or less the the same throughout the system.

This is the "fill" phase. If you excuse my crude MS Paint style diagram. This is also the first assumption I'm making. Fluid enters the side port, and by assumption there are orifices that allow it to fill the red chambers. It's a safe enough assumption, the fluid has to go somewhere and there's nowhere else really for it to go. Likely there's holes drilled in the side of the inner valve body. The pressure in the red chamber shuttles the valve core over in the direction of the blue arrow, allowing the flow to exit the valve and fill the wheel cylinders.

As the wheel cylinders fill and expand, flow slows down, and system pressure increases, something interesting happens. The outlet pressure (yellow) equalizes with the inlet pressure (red). 

 

This makes the valve core shuttle itself back closed, blocking off flow through the valve seat (the green boxes). This is the limit of pressure of rear braking. The valve core shuttles closed due to a common trick of hydraulic valves:

When you look at the valve core, you can see the diameter indicated by arrow 1 (yellow) is smaller than the diameter at point 3 (red). This means that the valve seat, flange portion, the thick bit at point 3 (green) is going to have a larger surface area on the outlet (yellow) side than the inlet (red) side. And because force due to pressure increases with surface area for the pressure to act on, the valve core will get shuttled towards the side with the lower force acting on it. As outlet pressure (yellow) increases towards equal with inlet (red) pressure, at a certain balance point the larger surface area pushes the valve closed, stopping flow and limiting total pressure available to the rear brakes.

 

So how does the adjustable proportioning come in? I'm going to make another assumption. I'm completing the spring in purple, and adding a spring seat in blue to push against and actuate the valve core. This adds a helper force to the inlet (red) pressure, helping to shuttle the valve open against the outlet (yellow) pressure. And as the external valve lever gets actuated, the cam rotates with the orange arrow, pushing the roller into the spring and compressing it, increasing the purple spring force. The more force coming from the spring, the more pressure you'll be able to put into the rear brakes.

 

So yeah. Hydraulics. Lots of fun. Hopefully this helps people understand how the valve works, and I didn't just waste a whole bunch of time because I'm bad at explaining things in an easy-to understand format.

Also the white arrow in the last picture is where I suspect the properly adjusted valve is supposed to leave the roller sitting on the cam, somewhere along that cam ramp, so that braking pressure doesn't drop off dramatically if the rear suspension is drooped for some reason, but so gentle increases in squat contribute to increases in rear brake pressure. The flat spot at roughly the 8 o'clock position on the roller would also support that being where it sat around ride height, with the constant motion slowly wearing down that one point.

 

 

 

 

 

 

[Salvagedcircuit]

53 minutes ago, gogmorgo said:

I stumbled across this thread earlier this week and have spent waaaaay too much time thinking about this. The fluid flow through the valve, I mean.

I have to make some assumptions based on not having the pieces at hand, but the result is a fairly standard sort of adjustable pressure regulating valve.

While thinking about how the valve works, remember there's two modes of operation of the valve.

The first dynamic mode is while fluid is flowing, when you step on the brake pedal and the wheel cylinders fill and push the shoes into the drums. Fluid pressure is not going to be the same through all parts of the system while fluid is flowing. Pressure will be lowest at the wheel cylinder end, and every little restriction, valve, orifice, kink in a line, is going to create a pressure spike as fluid resists moving through the restriction, up to the highest pressure area in the master cylinder.

The second mode is while you're holding steady pressure on the pedal and everything is static. The shoes have hit the drums and fluid is no longer flowing to expand the wheel cylinders. In static mode, pressure will be more or less the the same throughout the system.

This is the "fill" phase. If you excuse my crude MS Paint style diagram. This is also the first assumption I'm making. Fluid enters the side port, and by assumption there are orifices that allow it to fill the red chambers. It's a safe enough assumption, the fluid has to go somewhere and there's nowhere else really for it to go. Likely there's holes drilled in the side of the inner valve body. The pressure in the red chamber shuttles the valve core over in the direction of the blue arrow, allowing the flow to exit the valve and fill the wheel cylinders.

As the wheel cylinders fill and expand, flow slows down, and system pressure increases, something interesting happens. The outlet pressure (yellow) equalizes with the inlet pressure (red). 

 

This makes the valve core shuttle itself back closed, blocking off flow through the valve seat (the green boxes). This is the limit of pressure of rear braking. The valve core shuttles closed due to a common trick of hydraulic valves:

When you look at the valve core, you can see the diameter indicated by arrow 1 (yellow) is smaller than the diameter at point 3 (red). This means that the valve seat, flange portion, the thick bit at point 3 (green) is going to have a larger surface area on the outlet (yellow) side than the inlet (red) side. And because force due to pressure increases with surface area for the pressure to act on, the valve core will get shuttled towards the side with the lower force acting on it. As outlet pressure (yellow) increases towards equal with inlet (red) pressure, at a certain balance point the larger surface area pushes the valve closed, stopping flow and limiting total pressure available to the rear brakes.

 

So how does the adjustable proportioning come in? I'm going to make another assumption. I'm completing the spring in purple, and adding a spring seat in blue to push against and actuate the valve core. This adds a helper force to the inlet (red) pressure, helping to shuttle the valve open against the outlet (yellow) pressure. And as the external valve lever gets actuated, the cam rotates with the orange arrow, pushing the roller into the spring and compressing it, increasing the purple spring force. The more force coming from the spring, the more pressure you'll be able to put into the rear brakes.

 

So yeah. Hydraulics. Lots of fun. Hopefully this helps people understand how the valve works, and I didn't just waste a whole bunch of time because I'm bad at explaining things in an easy-to understand format.

Also the white arrow in the last picture is where I suspect the properly adjusted valve is supposed to leave the roller sitting on the cam, somewhere along that cam ramp, so that braking pressure doesn't drop off dramatically if the rear suspension is drooped for some reason, but so gentle increases in squat contribute to increases in rear brake pressure. The flat spot at roughly the 8 o'clock position on the roller would also support that being where it sat around ride height, with the constant motion slowly wearing down that one point.

Very neat explanation! I did not even realize the load sensing lever was adjustable until someone posted the fork adapter just earlier this week.

In some ways I naively discarded my load sensing prop valve when I upgraded to the WJ booster + master, but the amount of mud in the brake fluid and master makes me think it was still a wise choice.

[gogmorgo]

55 minutes ago, Salvagedcircuit said:

Very neat explanation! I did not even realize the load sensing lever was adjustable until someone posted the fork adapter just earlier this week.

In some ways I naively discarded my load sensing prop valve when I upgraded to the WJ booster + master, but the amount of mud in the brake fluid and master makes me think it was still a wise choice.

To be clear, the adjustability I mentioned in my post is only referring to the valve behaviour changing as the linkage to the axle moves through its regular travel. I think in hydraulic terminology you'd refer to this as a pressure limiting or pressure reducing valve, and then you would call it adjustable because you can change the spring tension to change the pressure drop through the valve. Because it adjusts itself during normal operation I think you'd be more properly calling it a variable pressure limiting valve, but we're getting into the weeds.

A typical adjustable valve wouldn't have the cam mechanism for the adjustment, usually they'd have a screw of some kind to increase or decrease spring tension manually, the way many aftermarket adjustable brake proportioning valves do it. But the cam and roller is a good way to make big adjustments quickly. Getting that much change in spring tension with a screw mechanism would be much more complicated, and likely not reliable enough to be constantly going back and forth going down the road.

(Note that a pressure limiting valve is different from a pressure relief valve, which relieves excess pressure by providing an exit path for fluid to prevent system pressure from building too much pressure. Hydraulics will usually relieve back to a reservoir, but some safety relief valves like on a propane or hot water tank will just dump to atmosphere... but again... weeds)

 

 

The adjustment with the "fork adapter" little bracket thing is less about adjusting the valve itself and more about adjusting the valve linkage. It repositions the linkage on the valve. You would have wanted to do that if you changed out the springs from factory, and wanted to make the valve worth with the existing linkage. The thread I believe you're referring to the OP was trying to find the correct orientation for the valve after a lift to set it up with an extended linkage. Confirming valve alignment to properly adjust an adjustable linkage.

It's also the thread I saw that prompted my own rabbit hole dive. And I think it links to this one, so I may as well link back to it.

I wouldn't necessarily recommend doing that particular adjustment where you pop the linkage off the valve itself and reposition the bushing and arm before stuffing it all back together. It looks to me like you're pressing a bushing with no splines onto a splined shaft, cutting new splines in the bushing at the same time. The manual recommends replacing the bushing every time you pull it off, and I'd be surprised if they were still available. Not that I actually checked beyond a quick google search just now... but the MoparPartsGiant places suggest it's been discontinued. Not surprising.

[zomeizter]

Thank you for the awesome info gog, my MJ is still equipped with this valve and even though I have discs on the rear axle, it feels like it is working properly. The brakes on this truck work excellent with stock vacuum booster and F100 master cylinder.